1. Field of the Invention
The present invention relates to image forming apparatus such as printers, copying machines, facsimile machines, and so on.
More particularly, the invention relates to improvement in image forming apparatus of the indirect (transfer) method or the direct method permitting variation in process speed for formation of image and variation in pixel density for formation of image, in which a desired image is formed and supported on an image bearing member such as an electrophotographic, photosensitive member, an electrostatic recording dielectric member, or the like by suitable image-forming process devices of the electrophotographic method, the electrostatic recording method, or the like.
2. Description of the Related Art
Conventionally, for example, as a method of charging the surface of the image bearing member as a body to be charged, such as the photosensitive member, the dielectric member, or the like in the image forming apparatus such as the electrophotographic apparatus, the electrostatic recording apparatus, and so on, it was common practice to employ the corona charging method being a non-contact charging method, in which a high voltage was applied to a thin corona discharge wire to generate a corona and in which the corona was made to act on the surface of the image bearing member to charge it.
In recent years, a contact charging method of keeping a charging member of a roller type, a blade type, or the like in contact with the surface of the image bearing member as a body to be charged and applying a voltage to the charging member to charge the surface of the image bearing member is going mainstream for reasons of low voltage processes, small ozone evolving amounts, low cost, and so on. Particularly, the charging member of the roller type is able to implement stable charging over long periods of time.
Here the charging member does not always have to be in contact with the surface of the image bearing member being the body to be charged, but the charging member may be placed in no contact with and in proximity to the image bearing member (proximity charging), for example, with a clearance (gap) of several ten xcexcm as long as a dischargeable area determined by a gap voltage and a corrected Paschen curve is ensured between the charging member and the image bearing member. In the present invention such proximity charging cases are also considered to be within the category of contact charging.
The voltage applied to the charging member may consist of only a dc voltage, but it is also possible to apply an oscillating voltage to the charging member to induce alternate, positive and negative discharges, thereby effecting even charging.
For example, it is known that when the oscillating voltage is applied in the form of superposition of a dc voltage (dc offset bias) and an ac voltage having a peak-to-peak voltage value not less than two times a discharge start threshold voltage (discharge start voltage or charging start voltage) of the charged object upon application of the dc voltage, the effect of averaging the charging of the charged body is achieved, so as to implement even charging.
The waveform of the oscillating voltage does not always have to be limited to only a sine wave, but may also be either of rectangular, triangular, and pulse waves. The oscillating voltage also embraces a voltage of the rectangular wave obtained by periodically switching the dc voltage on and off, and an output obtained by periodically changing values of the dc voltage so as to be equal to the superimposed voltage of the ac voltage and the dc voltage.
The contact charging method of charging the charging member by applying the oscillating voltage thereto as described above, will be referred to hereinafter as xe2x80x9cAC charging method.xe2x80x9d The contact charging method of charging the charging member by applying only the dc voltage thereto will be referred to hereinafter as xe2x80x9cDC charging method.xe2x80x9d
In the AC charging method, however, discharge amounts to the image bearing member (hereinafter referred to as a photosensitive drum) become larger than in the DC charging method. This was the cause of promoting deterioration of the photosensitive drum, e.g., shaving of the photosensitive drum or the like, and there were cases where an abnormal image such as image flow or the like was formed under a high temperature and high humidity environment because of discharge products.
In order to overcome this issue, it is necessary to minimize the alternate, positive and negative discharges, by applying the necessary and minimum voltage.
However, the relation between a voltage and a discharge amount is not always constant in practice, but varies depending upon the film thickness of the photosensitive drum, environmental variation of the charging member and/or air, and so on. Materials become dry under a low temperature and low humidity environment (L/L) to increase their resistances and resist discharge, so that the peak-to-peak voltage not less than a certain value becomes necessary for achievement of even charging. Even at the lowest voltage value to achieve even charging under the L/L environment, if the charging operation is carried out under the high temperature and high humidity environment (H/H), the materials will absorb moisture to decrease the resistances on the contrary and the charging member will cause more discharge than necessary. This will result in an increase in discharge amounts, which will pose problems of occurrence of image flow and blur, occurrence of toner fusion, shaving and life decrease of the photosensitive drum due to deterioration of the surface of the photosensitive drum, and so on.
In order to restrain this increase/decrease of discharge due to the environmental variation, the xe2x80x9cAC constant current control methodxe2x80x9d of controlling the current value of an alternating current flowing upon application of the ac voltage to the charging member was also proposed, in addition to the xe2x80x9cAC constant voltage control methodxe2x80x9d of always applying the fixed ac voltage as described above. According to this AC constant current control method, the peak-to-peak voltage value of the ac voltage can be increased under the low temperature and low humidity environment (L/L) where the resistances of the materials increase, whereas the peak-to-peak voltage value can be decreased under the high temperature and high humidity environment (H/H) where the resistances of the materials decrease. Therefore, it becomes feasible to restrain the increase/decrease of discharge, as compared with the AC constant voltage control method.
For aiming to further increase the life of the photosensitive drum, however, the AC constant current control method cannot be mentioned as perfect in order to suppress the increase/decrease of discharge amount due to variation of resistances caused by production dispersion and contamination of the charging member, capacitance variation of the photosensitive drum after long-term use, dispersion of high-voltage devices in the main body, and so on. In order to suppress this increase/decrease of discharge amount, it is necessary to employ means for decreasing the production variation of the charging member and the environmental variation and for canceling fluctuation of high voltage, which will increase the cost.
For stably providing high image quality and high quality over long periods of time, it is thus necessary to control the voltage and current applied so as not to cause over discharge and so as to implement even charging without a problem. As a method for realizing it, the inventors accomplished the invention of xe2x80x9cdischarge current amount control methodxe2x80x9d (Japanese Patent Applications No. 2000-11819 and No. 2000-11820), which is such a method that, where Vth stands for a discharge start voltage to the image bearing member upon application of the dc voltage to the charging member, during a non-image-forming period current values are measured upon application of at least one peak-to-peak voltage less than two times Vth and upon application of at least two peak-to-peak voltages not less than two times Vth and that a peak-to-peak voltage value of the ac voltage necessary for obtaining a desired discharge current amount to be applied to the charging means during an image-forming period is determined from the relation between the peak-to-peak voltages of the ac voltage and the alternating current values thus measured.
Since this method was configured to actually measure the relation between peak-to-peak voltages of the charging AC voltage and AC values and determine the peak-to-peak voltage value necessary for obtaining the desired discharge current amount, it became feasible to absorb the environmental variation, the production dispersion of the charging member, and so on.
This method is effective, especially, in the image forming apparatus of the cleanerless type without a cleaner such as a blade or the like used for cleaning up the region on the photosensitive drum by removing toner and others thereon. This is because the cleanup effect on the photosensitive drum is not expected in such apparatus, the condition is thus more severe for the image flow under H/H, and residual toner after transfer is not removed and will cause fog at the position of development due to charging failure at positions of transfer residual toner on the photosensitive drum unless an adequate discharge amount is given during execution of the charging process. In the image forming apparatus of the cleanerless type, as described above, it was necessary to control the discharge amount with higher accuracy, for using the ac voltage as the charging voltage.
In recent years, the image forming apparatus such as the printers and others are required to meet the necessity and resolution (pixel density) for printing on a variety of media such as thick sheets, OHP sheets, etc. with expanding diversity of user""s print needs, and it is met by providing a single apparatus with a plurality of process speeds (print speeds).
There arose, however, the following problems where the image forming apparatus using the contact charging apparatus for applying the oscillating voltage was adapted for a plurality of process speeds.
A) The first problem is interference fringes called xe2x80x9cmoire patternsxe2x80x9d, which appear when the frequency of the oscillating voltage (which will be referred to hereinafter as charging frequency) applied to the contact charging apparatus interferes with the spatial frequency of line pitch of line scanning.
A conceivable method for preventing this phenomenon is, for example, such countermeasures that the charging frequency fp is set sufficiently larger than the spatial frequency fs, but this method is not preferable because of the detrimental phenomenon of charging sound increasing with increase of the frequency, and others.
B) The second problem is periodical xe2x80x9cuneven development,xe2x80x9d which occurs when the frequency of the oscillating voltage applied to the charging member is close to an integral multiple or an integral submultiple of a frequency of an oscillating voltage applied to a developing sleeve.
This uneven development occurs when the frequency of the oscillating voltage applied to the charging member is around a frequency equal to an integral multiple or an integral submultiple of the frequency of the oscillating voltage applied to the developing sleeve. Since this is basically unevenness of surface potential on the photosensitive drum, discrimination of unevenness becomes easier in print of images with higher resolution. Therefore, the frequency range of occurrence of unevenness tended to become wider.
Particularly, in the case wherein the charging means and developing means are integrated into a process cartridge detachable from the main body of the image forming apparatus, electric paths for supplying the development bias voltage to the developing sleeve might be placed near electric paths for supplying the charging bias voltage to the charging roller from the structural aspect of contacts with the main body of the image forming apparatus. These paths could interfere with each other through a floating capacitance to produce beat components in the respective bias voltages, which can result in formation of an abnormal image similar to the uneven image described above.
C) The third problem is that if the charging frequency is fixed against change of the process speed the number of discharges in each unit surface of the photosensitive drum increases at a low process speed to promote the image flow and blur and the deterioration and shaving of the photosensitive drum under high humidity conditions while the number of discharges decreases at a high process speed on the contrary to fail to effect sufficient charging, posing the problems of uneven charging and charging failure.
This problem can be overcome by changing the charging frequency at a rate equal to a rate of the change of the process speed.
In order to solve the above problems, it was necessary to change the charging frequency against change of the process speed.
It becomes feasible to provide high-quality images in the process-speed-variable image forming apparatus, by combining the change of the charging frequency with the xe2x80x9cdischarge current amount control methodxe2x80x9d.
However, if the xe2x80x9cdischarge current amount control methodxe2x80x9d is applied with a change of the charging frequency against a change of the process speed as described above, the alternating current value will become smaller with a decrease of the frequency even at the same peak-to-peak voltage value of the oscillating voltage, whereas the alternating current will flow more with an increase of the frequency on the other hand. For this reason, the range of alternating current values measured becomes wider, which will increase the cost because of electronic parts used for accurate measurement in the wide range or which will degrade the measurement accuracy in measurement at low cost.
If xe2x80x9cdischarge current amount controlsxe2x80x9d are carried out at respective process speeds, the time will become longer for operations other than the print operation and this will result in degradation of usability.
An object of the present invention is to provide an image forming apparatus capable of optimizing the discharge current of the charging member.
Another object of the present invention is to provide an image forming apparatus that implements highly accurate, uniform, and satisfactory charging over long periods of time without increasing the cost, even in the case of images being formed at a plurality of process speeds.
Still another object of the present invention is to provide an image forming apparatus that can enhance the usability by decreasing the operation time for control.
Still another object of the present invention is to provide an image forming apparatus in which interference fringes are prevented from occurring.